1. Field of the Invention
The invention relates to a wear-restistant PVD-coating (Physical-Vapor-Deposition Coating) or Plasma-CVD coating (Chemical-Vapor-Deposition Coating) for a substrate such as a forming tool or a component thereof. The invention further relates to a method for applying the coating to a substrate.
2. Description of Related Art
Wear resistant hard coatings are used to improve the performance of forming tools, such as milling tools, punch tools, drills and other punching and cutting tools as well as components which are subjected to wear. Such coatings typically are thin (several micrometers) films, yet afford a considerable improvement of the wear resistance of tools and components to which the coating is applied. The hard coatings comprise one or several metalloids such as oxygen, nitrogen, carbon, boron, or silicon, and one or several metal elements. Despite the relatively high costs of these hard coatings, their use can be economically justified in certain applications by the significant improvement in wear resistance they offered.
These hard films typically are applied to substrate in a vacuum chamber in which a plasma, i.e. a particle vapor consisting of ions, is urged onto the substrate by an electrical field or with the aid of a magnetron field. These processes are also known as magnetron sputtering or sputter ion plating. The plasma is generated by impinging an electric arc, an electron beam, or a stream of rare gas ions onto a cathode, also known as a target, made of material from which the ions are formed. Depending on the process used, a liquid phase maybe formed during the plasma generation.
The vacuum chamber also may contain a reactive gas which becomes a component of the hard coating. If the plasma is created in a vacuum chamber into which the components of the hard coating are introduced in the gas phase, the process is as the known as the Plasma-CVD-process.
Initially, titanium nitrides were used to form the hard film. A marked improvement in wear resistance was achieved over high speed steels and hard metals. Further development of these hard coatings revealed that better results may be achieved when multi-metallic hard compounds are used. Examples of such multi-metallic coating include coatings of aluminium, titanium, and nitrogen, or of zirconium, titanium, and nitrogen. These coatings may be supplemented with other metals or metalloids to make the coating more stable.
Both the plasma-CVD coating process and the PVD coating process may yield an inhomogeneity at the edges of the coating. The conditions under which and the mechanism by which this inhomogeneity occurs are understood. Traditionally, coating process conditions were adjusted that a homogeneous coating was formed and the inhomogeneity was avoided, as it was believed that a homogeneous coating afforded superior performance.
Vaporized particles (ions) having the desired coating composition are urged from the vapor phase onto the substrate by application of an electrical field or with the aid of a magnetron field, which enhances the effect of a normal electrical field by superimposing a magnetic field thereon. The electrical field is generated by reducing the electric potential of the substrate below that of the plasma. The electrical or electrical/magnetic field has a higher charge density at the edges than in the center, i.e., at regions away from the edges. The higher charge density creates a stronger electrical field at the edge, which leads to a concentration of the particle flux in this area. The higher flux concentration causes the substrate to be more intensively hit at the edges by ions of all sorts, including the ionized particles of the hard film to be coated onto the substrate, target ions, and the carrier ions, such as argon ions, during target sputtering with the aid of carrier gas. These argon ions have the effect of compressing those particles of the hard film which have been deposited onto the substrate.
The edge effect caused by the electrical field is proportional to the strength of the electrical field applied. Therefore, known coating techniques utilize a relatively weak electric field to prevent inhomogeneity at the edges of the substrate. To compensate for this weak electrical field, the plasma in the vacuum chamber is very strongly ionized. Thus, a coating which is homogeneous throughout is obtained.